1. Field of the Invention
The present invention relates to a device and a method for generating a mixture of reducing agent and air and more particularly to such devices and methods used for posttreating internal combustion engine exhaust senses.
2. Description of the Prior Art
Because pollutant limit values have been set lower and lower in recent years, numerous devices and methods for posttreating exhaust gases in internal combustion engines have been developed. For example, by means of catalytic converter systems that use urea and/or ammonia as a reducing agent for NOx conversion, efficient exhaust gas posttreatments have been made available.
To achieve a reduction in NOx ingredients in exhaust gases, reduction catalytic converters have been developed especially for Diesel engines. A distinction is typically made between so-called SCR (Selective Catalytic Reduction) catalytic converters with urea metering systems and storage-type catalytic converters. The SCR catalytic converters are regenerated by means of a delivery of urea and/or ammonia reducing agent, while the storage catalytic converters are regenerated with hydrocarbons from the entrained engine fuel, in so-called rich exhaust gas phases.
From European Patent Disclosure EP-A 0 381 236, a system is known which, to remove nitrogen oxides from exhaust gases from a Diesel engine, meters in ammonia as a reducing agent. Also provided in this system is a turbocharger, which reduces the pressure of the exhaust gas. A urea-water solution employed is metered in by means of compressed air.
From German Patent Disclosure DE-A 44 41 261, a device for posttreating the exhaust gases of an internal combustion engine is known in which the catalytic converter capacity is meant to be improved via a metering device. The metering device is embodied as an electromagnetic metering valve, which moves (opens and closes) at the same clock frequency. The metering quantity is defined by the pulse-width repetition rate upon each stroke (and by the pressure difference). Adding the reducing agent to the exhaust gas system is preferably done as a function of a performance graph, that is, of the quantity and/or composition of the exhaust gas.
From German Patent Disclosure DE 42 30 056 A1, it is known to generate an aerosol on the basis of a reducing agent and compressed air acting upon it in a mixing chamber. The reducing agent and the air are delivered to the mixing chamber via separate lines. During one metering event, pressure fluctuations and turbulence can occur in the mixing chamber, which can cause a reverse flow of reducing agent, for instance an aqueous urea solution, into the compressed air line. During the metering operation, fluctuations in the volumetric flow of air can also occur. For instance, the volumetric flow of air drops with an increasing injection quantity of reducing agent into the mixing chamber. A dropping volumetric flow has an adverse effect, however, on the mixing together of reducing agent and air, as well as on the transporting of the reducing agent to the point of its injection into an exhaust system. If a urea-water solution is used, an undesired crystallization of urea from the solution can also occur in the line system.